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Wednesday, April 27, 2011

Making of a EC8020 LCR Phono Preamplifier, Part 1: Mechanical Construction


After the introduction of the magnificent EC8020 triode in the last post, I'm going to show the assembly process of a LCR RIAA phonostage based on this tube.

In the first part I will show the mechanical construction. It is a pure phonostage, no volume control. It is meant to be used with a separate line stage. It consists of two chassis, phono stage and external power supply unit. Construction is done in the usual style: 4mm anodized aluminum plates which carry all the parts. These will be mounted into wooden frames. Here a photo of the plates before construction begins:

The circuit is two stage, all transformer coupled, 600 Ohm LCR RIAA between them. Both stages use the EC8020 tube.

To control microphonics, each socket is mounted on a subassembly which is fixed to the main plate through rubber damping elements. The photos on the left show a single sub assembly element. The rubber elements come with 3mm threads on both ends. These get screwed into threaded metal blocks which in turn can be fixed to the metal plates.

The rubber elements have the right softness, not too hard and not too soft. If they are too soft, the sub assembly would wobble in the main plate and could act like a microphone, picking up the sound from the speakers of the system. If they are too hard, the damping effect is not there. As chosen, they don't pass vibrations from the main plate to the socket assembly. Any virations picked up by the tubes through the air is damped (converted to heat in the rubber). Although the EC8020s are not very microphonic, this damping system ensures a very quiet operation.

Since the small pates are electrically isolated from the main chassis by the rubber elements, a wire is added so that all metal parts are grounded. Otherwise the isolated metal plates could pick up stray fields and cause hum.

The next photo shows the main plate with all parts mounted. LCR RIAA modules in the front and 4 old stock paper in oil capacitors, one of them for each tube stage. The caps are manufactured by a company called Filtron. I found these to be superb sounding and just the right fit for a EC8020 phono stage.

The two black connectors are separate jacks for signal ground and chassis. With the switch in front of them both can be electrically connected or left unconnected. This gives maximum flexibility to adjust the grounding to any system configuration. Some turntables have separate ground wires from the disc platter which can be either connected to signal ground or the chassis terminal. The signal ground terminal is normally used for the tonearm ground wire.

The underside of the plate, everything ready to start wiring:

Interstage transformers, chokes and more capacitors will be mounted inside on a second tier.

The next two photos show the progress of the assembly of the power supply, both top and underside:

The main power transformer will be mounted on the top side and hidden under a transformer cover. Another transformer for the heater supply will be placed inside the chassis along with 4 chokes, 3 for B+ filtering and one for the heater supply.

The last photo shows the PSU plate mounted into an assembly jig which makes handling very easy and avoids the danger to scratch the plate while working on it.

Stay tuned for part 2 which will show the wiring.

Best regards


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